headcrabs are parasitic alien creatures that latch onto a host's head, forcibly taking control of their nervous system. They use their sharp appendages to pierce the skull and brain, effectively hijacking the victim's body. The host's face is obscured as the headcrab melds to it, and their body becomes a "zombie" under the parasite’s control. The victim’s humanity is overwritten, leaving them in a grotesque state with enhanced strength, aggression, and mindless obedience to the headcrab.